Referring to FIGS. 11 and 12, a conventional armrest assembly includes a first tube 100, a second tube 200 telescopically inserted in the first tube 100, an X-shaped lock 220 for locking the second tube 200 in position relative to the first tube 100 and an armrest 300 installed on the second tube 200. The first tube 100 includes a base 110 to be secured to a chair (not shown) and many pairs of apertures 160. The second tube 200 includes two upper apertures 211, two lower apertures 212, two opposite apertures 260 and a platform 290 for supporting the armrest 300. The X-shaped lock 220 includes two members 225 each including an aperture 223, an upper tip 221 and a lower tip 222. A spring 240 is compressed between the members 225 near the upper tips 221. Another spring 240 is compressed between the members 225 near the lower tips 222. The X-shaped lock 220 is put in the second tube 200. A pin 270 is inserted into the apertures 223 through the apertures 260 so as to keep the X-shaped lock 220 in the second tube 200. The upper tips 221 are inserted through the upper apertures 211. A button 280 is attached to the upper tip 221 of each member 225. The lower tips 222 can be inserted into one of the pairs of apertures 160 through the lower apertures 211 so as to lock the second tube 200 in position relative to the first tube 100. Via pressing the buttons 280, the upper tips 221 are retreated into the upper apertures 211, i.e., the lower tips 222 are retreated into the lower apertures 212 from the apertures 160 so as to the allow movement of the second tube 200 relative to the first tube 100. It is, however, difficult to simultaneously push the buttons 280 since they are located under the armrest 300 and on two opposite sides of the second tube 200. Moreover, the springs 240 eventually become inadequate to keep the lower tips 222 in the apertures 160. If this happens, the lower tips 222 can easily be jerked from the apertures 160 and worn against the second tube 200 without previously pushing the buttons 280 when the armrest 300 is lifted unintentionally.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in the prior art.